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The Ti–6Al–4V alloy was treated by inductively coupled rf plasma nitriding. The effects of plasma-processing time in the range of 5–35 min on the microstructure and the mechanical properties of the plasma-nitrided Ti–6Al–4V samples were studied. The plasma power input was adjusted at 450 W and pure N2 gas was introduced to establish a treatment pressure of 8.0–8.4 × 10−2 mbar. The characteristics of the nitrided layers have been investigated by microhardness testing, surface roughness measurements, optical microscopy, and x-ray diffraction. The results show that the surface microhardness increases as the plasma-processing time increases to reach 2000 HV0.1 at a plasma-processing time of 35 min. A high nitriding rate of 2.81 µm2 s−1 at a plasma-processing time of 25 min was achieved. The formation of the hard phases TiN, Ti2N, and Ti(N) in the Ti–6Al–4V surfaces are found to be the reason for the increased microhardness. Surface energy, yield strength and Young’s modulus for the nitrided Ti–6Al–4V alloy were calculated from the Vickers microhardness data.

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